A cement-modified soil (CMS) is a soil material that has been treated with a relatively small proportion of portland cement – less cement than is required to produce hardened soil-cement. The objective of the treatment is to amend undesirable properties of problem soils or substandard materials so that they are suitable for use in construction.
With the small quantities of cement generally used, CMS becomes caked or slightly hardened. However, it still functions essentially as a soil, although an improved one. The degree of improvement depends on the quantity of cement used and the type of soil. Therefore, by the addition of varying amounts of cement, it is possible to produce cement-modified soils with a wide range of engineering properties.
The improvement in engineering properties of a soil due to the addition of small quantities of cement can be measured in several ways including:
- Reduction in plasticity characteristics as measured by Plasticity Index (PI)
- Reduction in the amount of silt and clay size particles
- Increase in the California Bearing Ratio (CBR)
- Increase in shearing strength
- Decrease in volume-change properties
Cement-modified soils are usually classified into two groups according to the predominant grain size as follows:
- Cement-modified silt-clay soils are soils that contain more than 35 percent silt and clay (defined as material passing a No. 200 (75 µm) sieve in accordance with ASTM D4318). The general objective is to improve soils that are otherwise unsuitable for use in subgrades or subbase layers. Specific objectives may be to decrease plasticity and volume change characteristics, to increase the bearing strength, or to provide a stable working platform on which pavement layers may be constructed.
- Cement-modified granular soils are soils that contain less than 35 percent silt and clay (defined as material passing a No. 200 (75 µm) sieve in accordance with ASTM D4318). The usual objective is to alter substandard materials so that they will meet requirements specified for pavement base or subbase layers.
Cement-treated base (CTB) is an intimate mixture of aggregate material and/or granular soils combined with measured amounts of portland cement and water that hardens after compaction and curing to form a durable paving material. A bituminous or concrete wearing course is placed on the CTB to complete the pavement structure. CTB is widely used as a pavement base for highways, roads, streets, parking areas, airports, and materials handling and storage areas.
In CTB construction, the objective is to obtain a thorough mixture of an aggregate/granular material with the correct quantity of portland cement and enough water to permit maximum compaction. The completed CTB must be adequately cured to both let the cement hydrate and to harden the cement-aggregate mixture. The fundamental control factors for quality CTB are:
- Proper cement content
- Adequate moisture content
- Thorough mixing
- Adequate compaction
- Proper curing
The aggregate/granular material, cement, and water are typically mixed in a central mixing plant. Central plants can either be continuous-flow or batch-type pugmill mixers. CTB can also be mixed-in-place using transverse-shaft pulvermixers or traveling mixing machines.
Cement-treated base (CTB) thicknesses are less than those required for granular bases carrying the same traffic because CTB is a cemented, rigid material that distributes the load over a large area. Its slab-like characteristics and beam strength are unmatched by granular bases that can fail when interlock is lost. This happens when wet subgrade soil is forced up into the base by traffic loads. Hard, rigid CTB is practically impervious. It resists cyclic freezing, rain, and spring-weather damage. CTB continues to gain strength with age even under traffic. This reserve strength accounts in part for CTB's excellent performance.